Important fields of use of triethanolamine (TEOA) or its secondary products are, for example, soaps, detergents and shampoos in the cosmetics industry and also dispersants and emulsifiers.
For these and other fields of use, water-clear, colorless triethanolamine with the slightest possible discoloration, e.g. measured as APHA or Gardner color number, which retains these properties even over prolonged storage periods (of e.g. 6, 12 or more months) is desired.
A known problem is that a pure TEOA obtained by fractional distillation of a triethanolamine crude product which has been obtained, for example, by reacting ammonia with ethylene oxide has a yellowish to brownish or pink discoloration (color number e.g. about 10 to 500 APHA in accordance with DIN ISO 6271 (=Hazen)). This discoloration arises particularly in processes in which high temperatures are passed through.
During storage of the alkanolamine, even in a sealed pack and with the exclusion of light, this discoloration is further intensified. (See e.g.: T. I. MacMillan, Ethylene Oxide Derivatives, report No. 193, chapter 6, pages 6-5 and 6-9 to 6-13, 1991, SRI International, Menlo Park, Calif. 94025;
G. G. Smirnova et al., J. of Applied Chemistry of the USSR 61, pp. 1508-9 (1988), and Chemical & Engineering News 1996, Sep. 16, page 42, middle column).
The literature describes various methods of producing triethanolamine with improved color quality.
EP-A-36 152 and EP-A-4015 (both BASF AG) explain the influence of the materials used in methods of producing alkanolamines on the color quality of the process product and recommend nickel-free and/or low-nickel steels.
U.S. Pat. No. 3,207,790 (Dow Chemical Company) describes a method of improving the color quality of alkanolamines by adding a boron hydride of an alkali metal.
EP-A-1 081 130 (BASF AG) relates to a method of producing alkanolamines with improved color quality by treating the alkanolamine with hydrogen in the presence of a hydrogenation catalyst.
EP-A-4015 (BASF AG) describes that mono-, di- and triethanolamine with less discoloration are obtained by adding phosphorous or hypophosphorous acid or derivatives thereof before or during or directly after the stepwise reaction of ethylene oxide with ammonia and subsequent isolation by distillation.
WO-A-00/32553 (BASF AG) relates to a method of purifying TEOA produced by the reaction of aqueous ammonia with ethylene oxide in liquid phase under pressure and at elevated temperature by separating off excess ammonia, water and monoethanolamine from the reaction product, reacting the crude product obtained in this way with ethylene oxide and then rectifying it in the presence of phosphorous or hypophosphorous acid or compounds thereof.
EP-A-1 132 371 (BASF AG) relates to a method of producing alkanolamines with improved color quality where the alkanolamine is treated with an effective amount of phosphorous or hypophosphorous acid or compounds thereof firstly at elevated temperature over a period of at least 5 min (step a) and is then distilled in the presence of an effective amount of one of these phosphorous compounds (step b).